Stereophonic subcarrier signal generator



Jan. 25, 1966 A, H, BOT-|- sTEREoPHoNIc SUBCARRIER SIGNAL GENERATOR I NVENTOR. azf' #1A/s 5077 BY United States Patent iiiee 3,231,673 Patented Jan. 25, 1966 3,231,673 STEREOPHONIC SUBCAR SIGNAL GENERATR Adolf Hans Bott, Haddonfield, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 23, 1961, Ser. No. 146,785 8 Claims. (Cl. 179-15) This invention relates to signal transmission systems and more particularly to subcarrier wave `generators for adapting frequency modulation (FM) transmitters for the transmission of stereophonic sound signals.

The Federal Communications Commission (FCC) has recently authorized the transmission of stereophonic sound signals in the FM broadcast band. In the transmission of signals in accordance with the established standards, a matrix network is used to add and subtract stereophonc signals which may be derived from any suitable source such as spaced left and right microphones, to obtain a sum signal (L-l-R) and a dilerence signal (L-R). The difference signal (L-R) is used to amplitude modulate a subcarrier wave of 38 kilocycles (kc.) in a manner to produce a double sideband ysuppressed subcarrier. A pilot tone signal of 19 kc., which is used at the receiver to demodulate the subcarrier wave, the sum signal (L-l-R) and the subcarrier sidebands, are used to frequency modulate the main carrier wave which is transmitted. The transmitted signal is compatible with existing monophonic FM receivers since the sound from both Istereophonic signal channels (L-l-R) is demodulated and reproduced thereby. In addition, stereophonic receivers including demodulators for the main and sub-carrier Waves are able to reconstruct the original left and right stereophonic signals to reproduce sound in auditory perspective.

Heretofore the proposals which have been made for processing the original right and left channel stereophonic signals to provide the complex signals necessary to frequency modulate the transmitted carrier wave have ineluded complicated circuitry comprising a large number of electron tubes or transistors. These adaptors are expensive to build and correspondingly diflicult to adjust and maintain because of the large number of circuit elements.

Accordingly it is an object of this invention to provide a simplilied and relatively inexpensive stereophonic amplitude modulation double sideband suppressed subcarrier generator for FM transmitters.

A further object of this invention is to provide an improved and simplified circuit that is responsive to left and right stereophonic signals to matrix the left and right signals to produce sum and difference Isignals and to produce an amplitude modulated suppressed subcarrier wave corresponding to the difference signal.

Another object of this invention is to provide an improved stereophonic adaptor for FM transmitters including an improved circuit for developing and combining the amplitude modulated suppressed subcarrier sidebands corresponding to the difference between a pair of stereophonic signals, the sum of the stereophonic signals, and a pilot tone at half the Afrequency of the suppressed subcarrier.

A still further object of this invention is to provide an improved circuit for FM transmitters for the transmission of stereophonic sound signals which is simple and inexpensive to build, and which may be easily adjusted and maintained to provide reliable trouble free performance.

In accordance with the invention, the primary windings of four transformers are connected to form a bridge circuit. One of a pair of stereophonic signals is applied across one diagonal of the bridge, and the other of the stereophonic signals is applied across the other diagonal of the bridge. The sum of the stereophonic signals is developed ,across the primary windings of two of the transformers and induced into the second-ary windings thereof. These secondary windings are connected in series aiding relation to apply the sum signals to a sum signal output circuit.

The difference between the stereophonic signals will be developed across the primary windings of the other two transformers and induced into their secondary windings. The difference signal secondary windings are connected in series aiding relation, and form one input circuit of a balanced modulator, the other balanced modulator input circuit being coupled to the subcarrier wave signal source which, for example, in accordance with the established standards has a frequency of 38 kc. A subcarrier sideband output circuit is coupled to the balanced modulator to develop the amplitude modulated double sideband suppressed subcarrier signal representative of the difference signal. Thus an important feature of the invention is the provision of a balanced modulator circuit in a stereophonie subcarrier generator for an FM transmitter whose input circ-uit comprises a portion of a transformer matrixing network.

In accordance with a feature of the invention, the sum signal and subcarrier sideband output circuits are connected in series with a third output circuit. A pilot tone signal of 19 kc. which bears a ixed phase relation to the 38 kc. subcarrier wave is developed across the third output circuit. The composite output signal from the three series connected output circuits is then Ifed to the exciter of an FM transmitter. The series connected output circuits provide a simple and eflicient means for combining in the desired proportions, the sum (L-j-R) signals, the sidebands of the suppressed subcarrier signal as amplitude modulated by the difference (L -R) signal, and the 19 kc. pilot tone.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description, when Vread in connection with the accompanying drawings, in which the sole figure is a schematic circuit diagram of a stereophonic subcarrier signal generator for FM transmitters.

The stereophonic subcarrier generator has two pairs of input terminals 1l) and 12 for connection to suitable right and left channel stereophonic sound signal sources such as right and left microphones, stereophonic magnetic tape or stereophonic phonograph record pickups or the like. The stereophonic sub-carrier generator shown in the drawing performs essentially ve functions: 1) the preemphasis of the right and left signals; (2) the matrixing of the right and left signals to provide sum and difference signals; (3) the generation of a 38 kc. subcarrier wave and a 19 kc. pilot tone; (4) the double sideband suppressed carrier amplitude modulation of the 38 kc. subcarrier by the difference signal; and (5) the combining in the proper proportions of the sum signals, the subcarrier sidebands, and the 19 kc. pilot tone. The resultant composite output signal noted in v(5) above is developed at a pair of output terminals 14 which are adapted to be connected to the exciter of an FM transmitter, not shown.

In the following description of the stereophonic subcarrier generator, the subcarrier wave and pilot tone generating portions of the circuit will be considered lirst, followed by consideration of the matrix, the balanced modulator, and the combining circuits.

The only active devices used in the entire apparatus are two electron tubes (four tube sections) found in the sub- `carrier wave4 andY pilot tone generating circuits. Y TheV first tube section is a pentode connected in a crystal controlled oscillator circuit 16 which oscillates at 38 kc. 'The oscillator circuit'may be of any known, type providingisufficiently good frequency stability ofthe `generated lsubcarrier to satisfy theFCC standards. The -38kc. sig- "nal developed bythe oscillator circuit isifed .to-a second :tube -section ,which is connected tofoperate as a'buffer amplifier 18. The 38 kc, oscillator signal outputof the 'buiferlamplirier 18 is fed by way of the conductors'19v and 720.*to1'a balanced modulator circuit.

The 38-'kc...signal.from theoscillator circuit'16 is .also -fed to f axfrequency `divider stage 22 whichiincludes the ifthird andffourtlr.tube.` sections. VThe frequency .divider 2stage.22;is of fknowndesign andis adapted `to be *synchronized bythe appliedl 38V kc. signal to deliver an output signal*` of 19 kc.to a' resonant circuit 24 `which isftuned to 19a-kc. The tuned circuit '24 is coupled through .a transformer.26' to a variable resistorz'ZS. .The secondary winding of..the.secondaryltransformer 26 andthe resistorl 28 :comprise apilot tone output.,circuit 3th. Although the frequencyidivider stage '20 operates essentiallyas a muli'tivbrator. the `ringing action of the resonant.. circuit .24 f

- causes a-substantiallypure sine Wave to be developedand fed# to 1 the pilotV tone` output circuit:30.

The power supply for developing direct current (ll-C.) fplate voltage and alternating current (fn-C.) filament -voltage: for the four tube. sections includes af powertransrformer '32 .having aprimary winding tofwhich the power line An-C. voltageis applied. Thek power transformer hasv a first/secondary winding v34 which is connected to .a bridge. rectifierV 36gfor` developing the B-lvoltage'for the 'four tube sections. The B-lvoltagerisfilteredV bya resistance-capacitance.filter38 and applied toa shunt volt- 1 age regulator comprising a` Zener diode 40.' Since vthere 4arcano ampliierdevicesv in the other portions of the sub- .,carrier generator; substantially less.` filtering is requiredto prevent -60fand 120'cycle hum from being introduced in the signal channels. Filament'voltage for the tube sections Afis derived from a-` second secondary winding'42 on the `transformer`32, the-terminal on the winding 42 lmarked `Xvbeing connected'by conductors not shown to correspondingly:rnarked terminals of the tube filaments. A pilot light` 44 connected across thesecondary winding-E42 lindicates whenfpower is applied to the apparatus.

The'left and right audio frequency signals appliedto i the. input terminals 12 and 10 respectively Vare passed through suitable pre-emphasis networks 46 `andf48 which may be of anyy suitable design. The pre-emphasis networksf46 and 48 have a'time constant of about 75 microseconds, and operate to attenuate the higher audiofrequencysignals to a lesser-extent than the'v lower frequency audio signals. yAs is'known, the high frequency pre- -emphasis enables improved signal-to-,noise-ratio for the 'higher frequency signals reproduced at the receiver.

.Thefpre-emphasized-audio frequency signals from the networks '46 and 48 are fed respectively to coupling transformers'Stl .and 52, Vthe secondary windings of which-are ...coupledto la matrixing network for acldingrthe` left (L) signalgand the right'vsignal- (R) to produceV aV sum-.signal Y (L-'{-R),and` subtracting the 'left andright signals -to producea difference signaltL-R).

.Thematrixing network includes four transformers54, -156,158; and` 60; all having similar electrical' characteristics. As shown by the dotted linesI inthe drawing, theewindg. ings oflthe'rtransformersfl and;58 are on thexsannel core and may in actual practice be a single transformerihav- Jing twof separate; primary and twoseparate secondary .-windings. Inlikemannen the vwindings of the'transformers 56 and y60 are shownas being :wound onthesame .core.' The.transformers50-60'are well shielded to--prevent pickup of hum causedby stray sixtyl cycle fields.

The-primary windings of the transformers'S-l are -connected to form-a bridge. circuit;L having a` frstf-.pair of diagonal terminals A--A and a second pair of diagonal terminals, B-B. `The pre-emphasized left signal appearing at the winding 50a of .the transformer 50 is applied across the diagonal terminals A-A of the bridge, and the pre-emphasized right signalfrom the secondary Winding 52a..of the transformer 52 isfapplied'across the diagonal terminals B-B. AWith the connections as shown, .thesum signals are developed in the primary winding of the transformers 154 and 58 and-theaditference.signals are developed in the primary-.windings of the transformers 56 and 60.

To understand the ope-ration of they matrix network it should berno-ted that like-right 'and left signals produce 'like polarities of yvoltage at the upper terminals ofthe secondary windingstla andv 52aof the coupling transfo-rmers Si) and 52. f It can beseen by tracing the currentv o-w caused bythe like right and left-signals through the trans- Y :former 1154-60 .primary windings,` that these currents `add Vin the primaryfvwind-ings/of-the transformers 54 andSS and subtract inthe primary Vwindings of the transformers 55 and 50.

The-conneotionsfrom theprimary winding 50a to the bridge network includes a lresistorGZ which provides accurate impedance-matching. In like manner, a fixed resistor 64 and la' variable resistor'oonnects the secondary winding52a to the diagonal terminals -B-B of thebridgenetwork to provide 1 impedance matching. In lthe circuit shown, each of the primary windingsof the transformers 154-60 1'have a value of600I ohms,'thus.presenting an impe-dance `of -300fohms 'between 'the dia-gon-alterminals. The secondary windings 50a and' 52a of the circuit were ltapped to present a total .impedanc'eof '333 ohmsrso that the imped'ance'be exactly matched fr-oml the secondary vwinding 50a to the diagonal terminals A--A of the bridge lwith".-afresistance of 33 ohms for the resistorl. The ,same Ais true for thelsec'ondarywindings 52a to the diagonal terminalsfB-B of the bridge except'that the variable resistor fisraidjustable .to balance .thermatrixing network' for high frequency audio signals, such as .those of the order of l2V kc; which m'ay` have been unequally translated bythe 40 spre-emphasisriandiother coupling vcircuit-s.

Th'e 'Sum' (L-l-Rl-.signaldeveloped in the primary windwingoff'the: transformers r54f' and S8-isi induced in the .sec- '.Qndaify ewindingsrfiagand 58a. f"These,secondaryxwind- Y.-ings=-:are-.oonnecteddn series-aiding relationJtonpply the 45 S-total sum signal', toa timefdelay network'i.

In likem'anner thedierence fs-ign'al-isinduced in the .secondarywindingsr56ayand 60a of thetransformers 56 -and-60. ThefjwindingsiSga andi60'a arey connected in `*seriesaiding-relation. .across the diagonal terminal C-C 50 of -a bridge network lformedby` four diodes-70, 72, 74 and 7 6. :Four: matched diodes ,connectedas ,shown :may be purchased-commercially -;as .fa-.unit known as -a diode flquad. The othe1diagonal1D'-Dofthe diode bridge is :,conn'ected across the .primary-winding 7 8a` ,of fanzoutput 55 .-transformerVS-which may. have bifila-r wound primary and secondary windings. The primary winding-78a comprises two A. .likey 'portions which are connected' -together wby two parallel resistoinsfa fiXed-resistorfSt -andya resistor 8.2 havingan-adjustabletap.

yThe series connected secondary windingsfo'a and- `60a yof the VVmatrix network, :the diode; network, and -t-he output transformerS-are connected to operate `as a' ring modula- .tory Tothis end,`:38 kc, signals fromthe bufferampliier stage 18 -are'fedthro-ugh the conductor 20 to the variable 65 tapfon, the resistor-Land through the` conductor 19 to f Vthe.junction,off-tbesecondary-windings Stia-and 60a. The A 38.;kc..signal.amplitudeviszlarge relativeto that of the differencesignal (L-.R) ,vso-that the .-3 8; kc. :signalserves .ascswitching-voltage to control they oonductivitynofthe Ord-iodes 'Z0-76. y Whenrthe conductor 424) is driven positive .by: the138-.kc. Vsignal the diodes72and.-76-are conductive i 'andthe 'diodes A70; .and 7A-are 'reverse .-biased. When lthe conductorlt)A is driven Vnegativethe reverseis true in that -thefdiodesUZa-nd 76..;are cut otfan'd :the diodes f7@ and '74 75 conduct.

It will be seen that the switching action causes equal and opposite currents at 38 kc. to fiow in the two halves of the primary winding 78a, so that the 38 kc. subcarrier component -is balanced out. The tap on the resistor 82 may be adjusted to minimize any residual 38 kc. signa-1 that may appear in the secondary winding 78b of the transformer 78.

The signals appearing across the secondary winding 78h comprise the subcarrier double sidebands representative of a difference signal, and some higher order harmonic component-s. Accordingly, the secondary winding 7811 is coupled through a b-andpass filter network 84 to an output circuit 85 comprising a fixed resistor 8d connected in series with a resistor 88 with an adjustable tap. Assuming full range audio frequency input signals at the terminals and 12, i.e., 0-15 kc., the bandpass filter network is designed to pass signals in frequency range of 38 iro- L15 kc. or from 23 to 53 kc. The fitter removes the higher order components referred to above and in addition provides symmetrical response at both ends of the bandpass frequency range.

As mentioned previously, the sum (L-I-R) signals are applied to a time delay network 68. The time delay network delays the sum signal (L-l-R) to compensate for delays in the difference signal (L-R) through the bandpass filter network 84. The sum signal output circuit comprises a resistor 90.

A An important feature o-f the invention is the manner in which the time delay in the 'sum channel is matched to that which occurs through the bandpass filter network 84 in the difference channel. The bandpass filter 84 comprises in this embodiment a seven pole Tchebychev bandpass filter network having .01 db variation in amplitude response over about a 30 kc. range of the bandpass frequency response thereof. The arithmetic center frequency of the network 84 is 38 kc. and has a bandwidth of 36 kc. between points wherein the response of the filter is 3 db down. The design and construction of Tchebychev filters per se -is well known in the art.

VThe envelope del'ay of the bandpass filter 84, shown and described, is substantially constant for sideband signals resulting from the modulation of the subcarrier by audio signals up to about 9 kc. For sidebands resulting from audio signals labove 9 kc., the envelope time delay is a non-linear function of the frequency of the modulatingr signal.

Although it is possible to design the bandpass fiiter 84 to exhibit substantially constant envelope time delay for sidebands resulting from modulating signals over the enti-re audio frequency range of 0-15 kc., such a filter would have a wider frequency bandwidth, thus passing s ome of the higher orde-r components from the ring modulator. These components fall in a frequency range which maybe utilized for the transmission of another subcarrier, which may for example, have a subcarrier requency of 67 kc.

To maintain the proper left vs. right -channel separation as required byv the FCC Rules and Regulations, a signal of a given frequency in the sum channel must be delayed by the same amount as the difference signal sideband envelope which results from a signal of said given frequency, and the proper amplitude relation between the sum signal and the difference signal sidebands r'n-ust be maintained. The first requirement is complicated by the fact that the bandpass filter 84 does not have a linear time vs. frequency delay characteristic as noted above.

The time delay network 68 in the sum signal channel comprises a 7 'pole Tchebychev low pass filter having a .01 db variation in amplitude response over approximately kc. of its passband. The response of the delay network 68 is 3 db down at l18 kc. This provides a time delay vs. frequency characteristic in the sum signal channel which is complementary to that occuring through the bandpass network 844 in the difference signal channel as well as a substantially mat-ching amplitude vs. frequency response characteristic. In this manner signals of the same frequency in the sum and difference channels are delayed and attenuated by substantially the sa-me amount to provide the necessary left vs. right channel separation.

In the foregoing description, three output circuits have been described: the pilot tone output circuit 30 providing the 19 kc. pilot tone; the subcarrier sideband output circuit including double sideband amplitude modulated suppressed subcarrier signals in the range of 2.3-53 kc.; and the sum (L-l-R) signal output circuit having audio frequencies in the range of 0 15 kc. The pilot tone output circuit 30 and the subcarrier sideband output `circuit 85 are floating with respect to ground, but one side of the resistor 99 which is in the sum signal output circuit is connected to a point of reference potential indicated as ground. This feature permits simple and effective combining of the Various signals by connecting the three output circuits in series. To this end the high signal potential side of the resistor 90 is connected to one end of the resistor 86. The adjustable tap on the resistor 88 is in turn connected to one end of the resistor 28. One of the youtput terminals 14 is connected to the tap onthe resistor 28, .and the other output terminal is at ground potential. The amplitudes of the pilot tone and subcarrier sideband signals relative to the sum signal amplitude can be selected by adjustment of the taps on the resistors 28 and S8 respectively. Thus, the three output circuits are effectively connected in series between the output terminals 14, and the relative proportions of the three signals may easily be set by potentiometric adjustment.

The fact that the composite signal is available at the output terminals 14 of the subcarrier generator facilitates simple and direct measurement of the individual cornponents of the composite signal to enable proper proportioning thereof by adjustment of the taps on the resistors 28 and 88. The proper proportioning of the individual components of the composite signal is necessary to maintain left vs. right signal channel separation.

p The terminals 14 are adapted to be connected to the exciter of an FM transmitter, not shown, and the composite signal from the output terminals 14 is used to frequency modulate a carrier wave generated in the transmitter. In some existing monophonic transmitters, it may be necessary to modify the exciter circuit to extend its frequency response from 5 to 100,000 cycles. The reason for this is that the standards for the transmission of stereophonic signals requires 29.7 db left vs. right channel separation. For the L-i-R signal, a phase error of 3 from linear phase is permissible at 50v c.p.s. To meet this requirement, the low frequency response of the exciter should extend approximately to 5 c.p.s. For proper separation at the high frequency end of the audio range similar extensions in frequency response must be made with respect to the sideband representing the L-R signal. It is well within the skill of one working in the art to modify or design an exciter for an FM transmitter satisfying the above noted frequency response requirements. Although circuits in accordance with the invention are useful for adapting existing monophonic FM transmitters for the transmission of stereophonic signals, it should be understood that the invention is also applicable to stereophonic subcarrier signal generators for transmitters designed specifically to transmit stereophonic signals.

' What is claimed is:

1. A matrix circuit for a stereophonic system comprising a pair of input terminals one for each of a pair of stereophonically related signals, means providing four inductive windings connected to form a bridge circuit, means coupling one of said pairs of input terminals across one diagonal of said bridge circuit, means coupling the other of said pairs lof input terminal-s across another diagonal of said bridge circuit, means coupled to two of 7 saidv windings oppositely. connected 'in said bridge circuit 'for deriving. a signal corresponding to the sum of said 'stereophonically"related signals; and means coupled to the remaining two of said windings for deriving a signal corresponding to the difference between said stereophonically related signals.

2. A matrix and balanced modulator circuit for a stereophonic signal transmission system for a stereophonic system comprising a pair of input circuits one for ea-ch of a pair of stereophonically related signals, means providing four inductive windings connected to form a bridge circuit, means coupling one of said input circuits across one diagonal of said bridge' circuit, means coupling .the outer input circuit across another diagonal of said bridgeA cir-cuit, means coupled to two of' said windings oppositely connected in said bridge, circuit for deriving .a signal corresponding to the sum of said stereophonically related signals, means providing a balanced modulator including the remaining two of said windings, means providing a source of subcarrier waveenergy, coupled to said balanced modulator, and an output circuit for said balanced modulator.

3. A balanced 'modulator circuit for a stereophonic signal transmissionsystem for a stereophonic system comprising a pair of input circuits one for each of a pair of stereophonically related signals, means providing four inductive windings connected to form a bridge circuit, means coupling one of said input circuits across one diagonal of said bridge circuit, means coupling the otherv input circuit across another diagonal of said bridge circuit, a balanced modulator including means coupled to two of said windings oppositely connected in said bridge circuit for deriving a signal corresponding to the difference of said stereophonically yrelated signals, means providing a source of subcarrier wave'energy coupled to a balanced modulator, and an output circuit for said balanced modulator.

4. A stereophcnic subcarrier generator comprising a first 'and' secondv input circuits' for left and right stereo'- phonic signals respectively;

first, second, third and fourth transformers each 'having4 primary and secondary windings, a first series circuit comprising' the primary' windings of' said first and 'second transformers, a second' series' circuitv comprising` the primary windings of said third and fourth transformers, said' first and second 'series' cir cuits being connected in parallel so that said' third transformer primary winding is connected 'to said second transformer primary winding 'and' saidV fourth' transformer primary winding is 'connected to said first transformer' primary winding', means coupling said lfirst input circuit betweenv the junction of the primary windings of said first 'and fourth transformers'V and the junctionl of the primary windings of said'second' and third transformers, 'means coupling said" 'second' input' circuit between the junction `of the primaryy windings'of saidr'st and second transformers'andthe junction of the primary windings of saidthird and' fourth 'transformer"s whereby the difference' between said' left and right signals is. developed across said' second and fourth transformers primary windings;

` means connecting the secondary windings of said sec-, ond and fourtlr'transformers in series,`an output circuit Vincluding an" output transformer having primary and secondary windings, a plurality of uni-V directional conducting devices,'means connecting the secondary windingsv of' said secondv and 'fourth transformers, said unidirectional' conducting devices and the primary winding of said output transformer. as a balanced modulator,

and a' source of subcarrier wave energy coupled to' saidv balanced modulator whereby amplitude modulated suppressed carrier sidebands' representative` of the diie'rence between 'said' "left v'and right" signals are' developed across the secondary winding of said out put transformer.

5. A stereophonic subcarrier generator comprising a rst and second input circuits for left and right stereophonic signals' respectively;

first, second, third and fourth transformers each having primary and secondary windings, a first series circuit comprising the primary windings of said first and second transformers, a second lseries circuit: com- -prising the primary windings of said third and fourth transformers, said'first and vsecond series vcircuits being connected in parallel so that saidv third transformer primary winding is connected to'said second transformer primary Winding and said fourth transformer primary winding is connectedA to said first transformer primarywinding, means coupling said first'input circuit between the junction of the primary windings of said first and fourth transformers and the junction of the primary windings of said second and third transformers, means coupling said second input circui-t between the junction of the primary windings 'of'said first and second transformers and the junction of the primary windings of said third and fourth transformers, whereby the difference between said left an'd' right signals is developed across lsaid secondv and fourth transformer primary windings,

a ring modulator including means connecting the secondary windings of said second and fourth trans-v formers l in series, an output transformer having primary and secondary windings, a plurality of uni directional conducting devices, means connecting the secondary windings of said second and fourth transformers, said unidirectional conducting devices and theV primary windingv of said` output transformer vas a balanced modulator,

and a source of subcarrier'wave energycoupled between the junction of the secondary windingsI of said second and fourth transformers and a point sub-l stantially midway between the ends of the primary winding of said output transformer', whereby amplitude modulated suppressed carrier sidebands rep, resentative of the difference between' said left and right lsignals are developed across the secondary winding of said out-putV transformer.

6. A stereophonic ysubcarrier generator comprising aV first and second input circuits for left and right stereophonic signalsrespectively; matrix network means for combining saidv lefty and right signals to produce a differ-- ence signalrepresentative of the difference between saidy left and rightV signals and a sum signal representative of the sum of said left and right signals, a balanced'modulator including an input circuit coupled to said matrixmeans to receive said'dif'e'rence signal, means providing a source of subcarrier wave energy'coupled to said balanced modulator, an'output circuit for said modulator across which' i-'s developed amplitude modulated suppressed carrier side-V bands representative of said'difference signal, meanscou-y pled to said matrix network providing an output' circuit for said sum signal, means providingv a source' of pilot tone output circuit in series across said output terminals for said stereophonic' subcarrier generator'.

7. A'stereophonic subcarrier generator comprising a first and' second input circuits for left and' right stereo# phonic signals respectively;

first, second, third and fourth transformers ea'ch having primary and secondary windings, a first series circuit comprising the primary windings' of said first and second'transformers; a 'second 'series circuit A com prising the primary windings of said third and fourth transformers, said first and second series circuits being connected in parallel so that said third transformer primary winding is connected to said second transformer primary winding and said fourth transformer primary winding is connected to said first transformer primary winding;

means coupling said first input circuit between the junction of the primary windings of said first and fourth transformers and the junction of the primary windings of said second and third transformers, means coupling said second input circuit between the junction of the primary windings of said first and second transformers and the junction of the primary windings of said third and fourth transformers, whereby the difference between said left and right signals is developed across said second and fourth transformer primary windings and the sum of said left and right signals is developed across the primary windings of said first and third transformers:

ring modulator including means connecting the secondary windings of said second and fourth tran-sformers in series, first, second, third and fourth diodes each having an anode and a cathode, means connecting the first and second diodes in series across the secondary windings of said second and fourth transformers so that the cathode of the first diode is connected to the anode of the second diode, means connecting the third and fourth diodes in series across the secondary winding of said second and fourth transformers so that the cathode of the fourth diode is connected to the anode of the first diode and the anode of the third diode is connected to the cathode of the second diode, an output transformer having primary and secondary windings, said primary winding being connected between the junction of said first and second diodes and the junction between said third and fourth diodes;

source of subcarrier wave energy coupled between the junction of the secondary windings of said second and fourth transformers and a point between the ends of the primary winding of said output transformer;

and a bandpass filter network coupled to the secondary winding of said output transformer, said bandpass lter network having an output circuit across which is developed double sideband amplitude modulated suppressed carrier sidebands representative of the difference between said left and right signals.

A stereophonic composite wave generator having output terminals and comprising:

first and second input circuits for left and right stereophonic signals respectively;

matrix network including first, second, third and fourth transformers each having primary and secondary windings, said first and third transformers being Wound on one core, and said second and fourth transformers being wound on another core, a first series circuit comprising the primary windings of said first and second transformers, a second series circuit comprising the primary windings of said third and fourth transformers, said first and second series circuits being connected in parallel so that said third transformer primary winding is connected to said -second transformer primary winding and said fourth transformer primary winding is connected to said first transformer primary winding;

means coupling said first input circuit between the junction of the primary windings of said first and fourth transformers and the junction of the primary windings of said second and third transformers, means coupling said second input circuit between the junction of the primary windings of said first and second transformers and the junction of the primary windings of said third and fourth transformers, whereby the difference between said left and right signals is developed across said second and fourth transformer primary windings and the sum of said left and right signals is developed across the primary windings of said first and third transformers; a ring modulator including means connecting the secondary windings of said second and fourth transformers in series aiding relation for signals developed thereacross, first, second, third and fourth diodes each having an anode and a cathode, means connecting the first and second diodes in series across the secondary windings of said second and fourth transformers so that the cathode of the first diode is connected to the anode of the second diode, means connecting the third and fourth diodes in series across the secondary winding of said second and fourth transformers so that the cathode of the fourth diode is connected to the anode of the first diode and the anode of the third diode is connected to the cathode of the second diode, an output transformer having primary and secondary windings, said primary winding being connected between the junction of said first and second diodes and the junction of said third and fourth diodes;

a source of subcarrier wave energy having a frequency of 38 kilocycles, means for applying Wave energy from said source between the junction of the secondary windings of said second and fourth transformers and a point between the ends of the primary winding of said output transformer;

a bandpass filter network coupled to the secondary winding of said output transformer, said bandpass filter network having an output circuit across which is developed double sideband amplitude modulated suppressed carrier sidebands representative of the difference between said left and right signals;

means connecting the secondary windings of said first and third transformers in series aiding relation for signals developed thereacross, a time delay network coupled to said first and third transformer secondary windings to provide a delay substantiallyequal to the delay for signals through said ring modulator and bandpass filter, said time delay network having an output circuit across which is developed the sum of said left and right stereophonic signals;

a frequency divider coupled to said source of subcarrier wave, said frequency divider having an output circuit across which signals of half the frequency of said subcarrier Wave is developed;

and means coupling said bandpass filter output circuit,

said time delay network output circuit, and said frequency divider output circuit in series across the output terminals of said stereophonic composite wave generator.

References Cited by the Examiner UNITED STATES PATENTS 2,093,540 9/1937 Blumlein 179-l00.4

DAVID G. REDINBAUGH, Primary Examiner. 

1. A MATRIX CIRCUIT FOR A STEREOPHONIC SYSTEM COMPRISING A PAIR OF INPUT TERMINALS ONE FOR EACH OF A PAIR OF STEREOPHONICALLY RELATED SIGNALS, MEANS PROVIDING FOUR INDUCTIVE WINDINGS CONNECTED TO FORM A BRIDGE CIRCUIT, MEANS COUPLING ONE OF SAID PAIRS OF INPUT TERMINALS ACROSS ONE DIAGONAL OF SAID BRIDGE CIRCUIT, MEANS COUPLING THE OTHER OF SAID PAIRS OF INPUT TERMINALS ACROSS ANOTHER DIAGONAL OF SAID BRIDGE CIRCUIT, MEANS COUPLED TO TWO OF SAID WINDINGS OPPOSITELY CONNECTED IN SAID BRIDGE CIRCUIT FOR DERIVING A SIGNAL CORRESPONDING TO THE SUM OF SAID STEREOPHONICALLY RELATED SIGNALS, AND MEANS COUPLED TO THE REMAINING TWO OF SAID WINDINGS FOR DERIVING A SIGNAL CORRESPONDING TO THE DIFFERENCE BETWEEN SAID STEREOPHONICALLY RELATED SIGNALS. 